Hantaviruses cause two highly lethal human diseases, hemorrhagic fever with renal syndrome (HFRS) and hantavirus pulmonary syndrome (HPS), and have the potential to be used as bioterrorism agents. Hantaviruses replicate predominantly in endothelial cells but cause no apparent damage to the infected endothelium. Non-pathogenic hantaviruses also infect human endothelial cells, indicating that intracellular events determine the capacity for hantaviruses to cause disease. Using DNA arrays we have found that the non-pathogenic hantavirus Prospect Hill Virus (PHV) rapidly induces interferon (IFN) responses within endothelial cells that are not elicited by pathogenic Hantaan virus (HTNV) (HFRS) or New York-1 virus (NY-1V) (HPS). PHV replication is consequently inhibited while HTNV and NY-1V replicate within human endothelial cells. In addition, co-infection of PHV and NY-1V reduced PHV-directed IFN responses, suggesting that NY-1V negatively regulates defensive cellular responses. These findings indicate that pathogenic and non-pathogenic hantaviruses differentially regulate innate cellular defenses and suggest that the ability of pathogenic hantaviruses to alter endothelial cell responses is a fundamental determinant of hantavirus pathogenesis. Although there is no information about how hantavirus proteins might direct or regulate IFN responses, we recently identified hantavirus protein interactions that may influence IFN responses. Our preliminary results suggest that pathogenic hantavirus proteins play a role in blocking cellular defenses and thereby permit pathogenic hantaviruses to replicate within endothelial cells. Objective: We propose to investigate the differential regulation of endothelial cell responses by pathogenic and non-pathogenic hantaviruses. We will define hantavirus proteins that repress IFNb responses and determine the role of IRF-3 and NF-kB activation on hantavirus replication in order to define signaling pathways and proteins that are regulated by pathogenic hantaviruses. These experiments provide a fundamental understanding of hantavirus regulation of innate cellular defenses, define determinants of hantavirus pathogenesis and are anticipated to provide therapeutic targets for hantavirus disease interventions. Specific Aims: Aim 1) Define Hantavirus Proteins that Regulate Cellular IFN Responses. Aim 2) Evaluate Mechanisms of Signaling Pathway Regulation by Pathogenic Hantaviruses.